On the Lines of the Solar Spectrum

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[ 149 ]

IX. On the Lines of the Solar Spectrum.
By Sir David Brewster, KH., D.C.L., F.RS. L.&&, and Dr. J. H. Gladstone, F.RS.

Eeceived January 26, — Eead February 23, 1860.

In a paper published in the Transactions of the Royal Society of Edinburgh for 1833,
Sir David Brewster stated that by various means he had examined the lines of the
solar spectrum, and those produced by the intervention of nitrous acid gas, and had
delineated them on a scale four times greater than that employed in the beautiful map
of Fraunhofer. Some portions also, which were more particularly studied, had been
drawn on a scale twelve times greater. " Fraunhofer," he continued, "has laid down
in his map 354 lines, but in the delineations which I have executed, the spectrum is
divided into more than 2000 visible and easily recognized portions, separated from each
other by lines more or less marked, according as we use the simple solar spectrum, or
the solar and gaseous spectrum combined, or the gaseous spectrum itself, in which any
breadth can be given to the dark spaces." None of these drawings, however, were
published at the time. Frequent observations were continued during the years 1837,
1838, and 1841 ; and now, after a lapse of many years, the various delineations, having
been collated and arranged by Dr. Gladstone, form the principal diagrams in the Plate
accompanying this paper.

Fig. 1 of Plate IV. represents the lines observed when the sun was at a considerable
altitude above the horizon, and its light was examined by means of a good prism
and telescope. The spectrum is delineated on so large a scale that it was necessary
to divide it into two portions, the upper diagram representing the part between the
least refrangible end and the line designated F 7, the lower diagram the part between
F7 and the most refrangible end. On a comparison with Fraunhofer's large map*,
the principal lines and features will be easily recognised ; but it will be seen that
every portion of the spectrum contains lines wanting in the earlier drawing, and that
parts which Fraunhofer has marked by one line are resolved into groups of bright
spaces alternating with dark lines. The figure of the spectrum extends at the more
refrangible or violet end to about the same distance as that of the Bavarian philosopher,
but it exhibits a considerable extension at the red or less refrangible end. The
principal lines are indicated by those letters, A, a, B, C, &c, which were assigned to
them by him, and the larger intermediate lines are marked by numbers, 1, 2, 3, &c,
beginning afresh on the more refrangible side of each letter ; so that any one of these
may be expressed by a combination of a letter and numeral; as, for instance, C6, a

* In his " Bestimmung des Erechungs- und Parbenzerstreuungs-Vermogens verschiedener Grlasarten."
MDCCCLX. X

150 SIE DAVID BEEWSTEE AND DE. J. H. GLADSTONE ON

remarkable line in the orange, of which much will be said hereafter. The extreme
violet is lettered, both in this and in a map to be subsequently described, by that con-
tinuation of the alphabet which has been adopted by M. BecquereiA It was necessary
to indicate in some similar manner the newly published, though not newly discovered,
lines at the red end of the spectrum ; and as the alphabet has not been appropriated by
M. Becqueeel beyond P, and it is not likely that further research will largely extend
the spectrum in that direction, it was thought safe to take the end of the alphabet, and
denoting the first strongly-marked line before A by Z, to work backwards into those
slightly refrangible rays, which have been as yet unresolved by human vision. Some of
the dark spaces of the spectrum are of an appreciable breadth, in which case they are
represented as bands ; and where the observation of a line was indistinct or uncertain, it
is marked by an interrupted instead of a continuous line.

The light less refrangible than A is red but extremely faint, so faint indeed, that few
observers of the spectrum have perhaps ever seen it; and the only drawing hitherto
published of lines in it, appears to be in a map of the solar spectrum by M. Matthiessen
of Altonaf. He represents a few lines which, on comparison with fig. 1, may be iden-
tified as the band anterior to Y, Y itself, and the band Y 1. In order to map the lines
and bands in this portion of the prismatic image, Sir David Brewster was obliged to
take extraordinary precautions. The telescope was lined with black velvet, in order to
exclude any reflected light ; a low power was employed ; the slit was made about the
8th or 10th of an inch wide, and the eye of the observer was washed with water to
cleanse the fluid that lubricates the cornea J. The most prominent line in this space is
that marked Y.

The red space between A and B is marked by Fraothofer merely by a bundle of
lines midway between the two, and designated a. It is indeed difficult to resolve the
light that extends from A to #, but between a and B lines and bands are easily per-
ceptible. This space is delineated not only in fig. 1, but on a larger scale in fig. 2, while
fig. 3 is a still more magnified view of the bundle of lines that constitute a. The
succession of pale thin bands between A and a 1, represented in fig. 2, were only
distinctly seen on one occasion ; and to the drawing Sir David Brewster appends the
remark, "their exact places and breadths require to be better fixed." They usually
present themselves to the observer as two or three broader bands, filling up nearly the
whole space. The series of bands marked by faint lines between a 3 and B, is a peculiar
feature of this part of the spectrum.

Between B and C little can be detected with certainty beyond the four lines previously
observed by Fraothofer. The orange space between C and D is far richer ; but imme-
diately beyond the double line D is a yellow space of considerable breadth marked by
only one, and that a faint line, and this is the most luminous portion of the whole
spectrum.

* Bibl. Univ. de G-eneve, xl. f Referred to in Compt. Bend. xix. p. 112,

% See Comptes Eendus, 1850, torn. xxx. p. 579.

THE LINES OF THE SOLAE SPECTEUM. 151

The green space between E and F was made the object of special study, and a sepa-
rate map of it on a scale twelve times that employed by Feaothofee is given in fig. 4,
while fig. 5 represents, on a still larger scale, the lines included within what is ordinarily
denominated the triple line b.

The blue space between F and G is that of which the delineations appear the least
trustworthy ; indeed there is some discrepancy among the drawings of the lines between
F18 and F27. The violet space from G to H is probably deserving of greater con-
fidence. The sparse lines beyond H seem to extend to the further limit of Becqueeel's
group L, which, with the group I, is faintly indicated.

In the summer of 1858 Dr. Gladstone examined the two extremities of the solar
spectrum where Feaothofee's map is manifestly deficient ; and as these are the portions
where there is least light, he made his observations on very bright days about midsummer,
and at noon, consequently when the sun was at about its highest in the latitude of
London. The instrument of the Eev. Baden Powell, described and figured in the
British Association Report for 1839, was employed, and a good prism of flint-glass
having an angle of 45°. The light of the sun was reflected from the quicksilvered
mirror of a heliostat ; and a blue cobalt glass was placed in front of the eyepiece of the
telescope. This latter precaution is necessary in order to intercept, or greatly reduce
in brilliancy, the orange, yellow, and green rays ; for although the telescope takes only
a part of the prismatic image into the field of view at a time, it will still happen that,
however pure the glass of the prism may be, and however carefully cross-lights are pre-
vented, some of the middle rays will be irregularly dispersed, and will mix with the red
or lavender ; and when the sun is shining brightly into the instrument, these rays will
be in sufficient quantity to render it a matter of great importance that they should be
stopped by cobalt glass. The results of these observations are given in fig. 6, which is
on the same scale as Fraunhoper's spectrum, or one-fourth of that adopted in fig. 1.

From this map it is evident that the appearance of lines and bands between A and B
is not confined to periods of the day, or latitudes, where the sun is at no great height
above the horizon, although none, except the group a, are delineated in the Munich
diagram. It will presently be seen, however, that they become much more visible when
the sun's light traverses a larger portion of our atmosphere. Light was perceived by
Dr. Gladstone for a considerable space anterior to A, but no lines were distinguishable
in it, on account probably of the inferiority of his apparatus to that employed by Sir
David Beewstee, whose drawings also had not been seen or heard of by him when he
made these observations.

On comparing this map in its delineation of the rays beyond H with the drawings of
M. Becqueeel* and Professor Stokes f, it will be at once evident that it contains the
lines marked by them as far as it extends, besides many finer ones. Yet the three
maps represent three different phenomena: that of Becqueeel the lines and spaces
where there is no chemical action ; that of Professor Stokes the lines and spaces where
* Bibliotheque Univ. de Geneve, xl. t Philosophical Transactions, 1852.

x2

152 SIE DAVID BEEWSTEE AND DE. J. H. GLADSTONE ON

no fluorescence is produced in bisulphate of quinine, or similar bodies ; while this repre-
sents those where there is no effect on the organ of vision. Professor Stokes mentions
in a note to his paper, that he had " succeeded, by a particular arrangement, in seeing so
far into the ' lavender ' rays as to make out the groups of fixed lines m, n, p by means
of light received directly into the eye, and even to perceive light beyond that." The
m, n of his drawing answer to the M and N of M. Becquerel's map, and his p is nearly
coincident with its termination. The figure of M. Matthiessek' extends at least as far
as the fluorescent spectrum of Professor Stokes, but it does not clearly appear how it
has been obtained.

Professor Piazzi Smyth, during his recent astronomical experiment on the Peak of
Teneriffe, had an opportunity of analysing the light of the sun when seen through a
smaller amount of atmosphere than has fallen to the lot of any other investigator. He
has incorporated such observations made near noon-day in the account of his experi-
ment*, but unfortunately his apparatus was imperfect. His statements amount to this:
that " comparing a high sun-spectrum at the sea-level with a similar one at Alta Vista
10,702 feet high, both observed direct, it was found that while in the former the
spectrum terminated immediately beyond H, and the two bars of H were nebulous, in
the latter the spectrum extended beyond H to three times the distance of its bars
asunder: the two said bars also lost all their nebulosity, being clearly resolved into
their component lines; many fine clear lines were seen between them, and many
appeared nebulously in the space beyond." He gives other good reasons also for the
conclusion that "we may assume that there is a much greater amount of the more
refrangible rays in the sun's light in the upper, than in the lower regions of the atmo-
sphere." To his drawings of the spectrum at evening, reference will be hereafter made.

Absorption hy the Atmosphere.

The absorbent power exercised by the earth's atmosphere on the more refrangible
rays already alluded to, is extremely evident when the sun is shining through a long
reach of air, as at his rising or setting. Everyone has remarked that under such
circumstances the luminary acquires a more or less red colour. This is due to the
total absorption of the lavender and violet rays, the greater or less absorption of the
blue and green, and the diminution of the yellow and perhaps the orange rays, whilst
the red pass apparently unimpeded through the deepening atmosphere, and from the
absence of the dazzling yellow, reveal themselves with their characteristic lines to the
eye even when unshielded by any coloured medium. This is also the case, to a certain
extent, when the sun at any altitude shines through mist or smoke ; and the same result
may be at any time obtained by placing in the path of the rays a vessel of water with
which a few drops of milk have been mixed.

It is scarcely necessary to say, that, though the visible spectrum becomes shorter as the

# Philosophical Transactions, 1858 ; pp. 503 to 507.

THE LINES OE THE SOLAE SPECTEUM. 153

sun sinks, there is no real contraction of its several parts, the distance between the fixed
lines measuring precisely the same at different hours.

The red sun is often attended with a blue sky ; the diffused light, in fact, always
exhibits portions of the spectrum which are wanting in the direct rays. This may give
rise to a singular phenomenon. There is a green glass coloured by means of copper and
iron which has the power of absorbing the whole of the red and orange rays, but allow-
ing others to penetrate. If the landscape be viewed through this, all the ordinary
objects are seen with tolerable distinctness, though of a greenish or bluish colour, but
anything purely red is extinguished ; and it has happened to Dr. Gladstone, on
examining the western sky near sunset with such a glass, that he has seen the various
configurations of the clouds and of the horizon perfectly, whilst the sun itself was so
little visible that its presence would not have been detected, unless it had been specially
sought for. No black space is produced in this experiment ; for of course that portion
of the sky immediately between the solar disk and the spectator is sending diffused light
like the parts adjacent.

Atmospheric Lines.

Beyond this absorption of the more refrangible rays, another and more remarkable
phenomenon presents itself when the sun descends towards the horizon and shines
through a rapidly increasing depth of air. Certain lines which before were little
if at all visible, become black and well-defined, and dark bands appear even in what
were formerly the most luminous parts of the spectrum. This has been observed both
at sunrise and sunset ; and it is not necessary that the luminary should be just on the
horizon, or that the absorption of the more refrangible rays should be very complete ;
indeed these atmospheric lines have been seen when H and k were easily distinguishable,
and light was perceptible far beyond. Fig. 7 is a map of these lines and bands, com-
piled from the independent drawings of the two authors, which agree very closely. In
calling them " atmospheric, 5 ' nothing more is meant to be expressed by the term than
the mere fact that these lines or bands become much more visible as the sun's rays pass
through an increasing amount of atmosphere. Sir David Bkewster first observed them,
and gave a verbal description of them in the paper already referred to *, but his more
exact and extended observations were made subsequently. In the least refrangible part
of the spectrum which was first observed by him, the bands X3, Yl, Y3, and Zl become
very dark as the sun sets, and assume the appearance of broad black bands, supporting
on each side the finer lines Y and Z. A becomes very wide even when the sun is at a
considerable altitude, its aspect being that represented in fig. 8, which is on the same
scale as fig. 3 ; and gradually the lighter portions on each side of the black line become
quite dark, and the series of lines before it are converted into a dark band, so that it
presents the appearance of two black spaces divided by a narrow luminous space ; but as
the sun sets this light also disappears, and it becomes one uniform expanse of darkness.
The group a becomes much deeper in shade, but not uniformly so, for the bright spaces
between the bands are not absorbed. In general the shaded parts between A and B
# Edinburgh Philosophical Transactions, vol. xii. pp. 529, 530.

154 SIE DAVID EEEWSTEE AND DE. J. H. GLADSTONE ON

become rather less luminous as the sun descends ; but the greatest effect is observable in
the narrow band a 8, the nearest to B, the appearance of which at that time is repre-
sented in fig. 9, where the scale is double that of fig. 2. C and most of the lines between
it and C6 are deepened; and the last-mentioned line is particularly noticeable, as it begins
to intensify when the sun is at a considerable altitude; indeed in Great Britain it is
very evident during winter at any hour of the day. When the sun is about to set, or
when it is just rising, it is one of the most strongly marked lines in the whole spectrum.
C 15 increases to a black band, C 16 becomes more evident than before, and the double
line D is very strongly developed ; D 1 deepens in shade ; and at about D 2 commences
a band, marked by the Greek letter § in the diagram (fig. 7), which is one of the most
characteristic features of the prismatic image of light that has passed through a long
space of air. It is discernible in the diffuse light of a dull day at any hour; it is that
which Professor W. A. Miller observed manifesting itself on the occasion of a thunder
shower*; and it becomes evident in the direct solar rays when the luminary is several
degrees above the horizon : from its occurrence in a most luminous part of the ordinary
spectrum, and from its great breadth, it intercepts a large amount of light, even at that
time ; and when the sun is just setting, it becomes a broad space of almost total darkness.
It appears to cover a larger amount of the image in the direction of E, as it deepens in
shade. It is succeeded by other bands, e and £, and by a line ??. The latter becomes
very prominent, and from its vicinity to the comparatively faint line E, which is not per-
ceptibly deepened by the atmosphere, has sometimes been mistaken for E itself. Beyond
h there are several remarkable bands, especially those designated in the map by the letters
/ and sc. F itself seems to become nebulous ; and between it and G appear seven bands,
X, (*, v, |, o, t, g : the representation of the six last given in the map is on the authority
of one single observation and drawing by Sir David Brewster. It is but rarely that
this part of the spectrum can be seen at sunset ; and the portion beyond G is so seldom
visible, and when seen is so faint, that no atmospheric lines or bands have been descried
in it.

Allusion has already been made to the fact that the band & is easily recognizable in
diffused light ; and this is more or less true, as might be expected, of the other bands
and lines. Indeed the western sky after sunset affords a favourable opportunity for
studying those in the more luminous parts of the spectrum. If the sky be red, C, C 6,
D, and & generally appear as four very black bands ; if it be yellow, they are not so well
defined. The various colours of the clouds also afford a great variety of appearance, but
not so much in the character of the dark spaces, as of the luminous image which they
intersect. There seems to be a difference in the visibility of these bands at different
times, which is not readily accounted for; thus on October 29, 1837, at Allerly, near
Melrose, at the instant of sunset the luminous sky gave a spectrum in which C6, though
distinctly seen, was not black, nor was D, nor §, while the line B was very broad and
deep. For thirty-seven minutes after sunset this black B was discernible; but even
then, and indeed until the twilight had gone, the forementioned bands, usually so

# Philosophical Magazine, August, 1845, p. 85.

THE LINES OF THE SOLAE SPECTBTJM.

155

prominent, did not appear either black or wide. On October 31, again, the atmo-
spheric lines were not so dark as usual, while the rays beyond C 10 had evidently
suffered a considerable absorption. On neither of these occasions was there much
colour in the sky ; but that the phenomena did not depend on either the absence or
presence of humidity in the atmosphere, is evident from the fact that on the earlier date
there was a keen frost, while on the later day the weather was wet, the thermometer
being 38° F. at the time of observation. That moisture, however, has some influence
in the production of these bands, is shown by the effect of a fog on the solar radiations ;
thus on November 20, 1858, at 10 o'clock a.m., at London, the sun loomed red through
a mist, and a prismatic analysis of its light showed a and B with extreme distinctness,
and the characteristic C 6, 5, and n*

When the sun's rays traverse a mixture of milk and water, though they are dispersed
and absorbed to a large extent, especially at the more refrangible end of the spectrum^
these atmospheric lines are in no way exhibited ; a proof, if additional proof be needed,
that they are not owing to the mere reduction of the light.

It is a most beautiful and striking sight to observe the gradual appearance of these
characteristic lines as the sun descends towards the horizon. Professor Piazzi Smyth
remarked it ; and from his elevated position on the Peak, he had the peculiar advantage
of observing the sun when it had sunk beneath the astronomical horizon, even to the
depth of 1°*1. In his drawings* it is curious to trace the gradual darkening of a and B,
and the intermediate lines, of what he calls the " growing " line beyond C, and of the
dark band that follows D. In the drawing made by him when the sun was at an altitude
of — 1°"1, the bands between C 6 and b appear to occupy a still larger space than in the
diagram compiled from the observations in Scotland and England. This, of course, is
in accordance with what might have been anticipated.

Measurements.
The following are the refractive indices of the principal lines of the spectrum, atmo-
spheric or otherwise, as determined for the flint-glass of which the prism employed by
Dr. Gladstone is composed : —

F ..... . 1*6292

G ..... . 1-6404

G33 1-6464

H 1-6501

K ..... . 1-6513

A . .

1-6069

a . . .
B . .

1-6087
1-6104

C . .

1-6122

C6. .
D . .

. 1-6139
1-6162

End of &

. 1-6192

End of £
E . .

. 1-6219
. 1-6234

b . .

. 1-6249

I . . . . .

L . . ...
2nd of group M .
5th of group M .
1st of group N .
Philosophical Transactions, 1858, Plate XXXV.

1-6548
1-6567
1-6589
1-6614
1-6642

156 SIE DAVID BEEWSTEE AND DE. J. H. GLADSTONE ON

It may be well to state that the angular measurements from which these numbers
were calculated, showed a proportion between the different lines almost identical with
that represented in the drawings of Sir David Beewstee, and also in the map of
M. Matthiessen of Altona,

Light reflected from the Moon.

As the light of the moon is only that of the sun reflected from her surface, it might
be anticipated that it would exhibit the same fixed lines ; and so indeed it does. The
authors of this paper, like other observers*, have remarked this. Sir David Beewstee
saw distinctly the lines in the lunar spectrum from B to near H, and placed it on
record that they were exactly the same as those in the solar spectrum. Dr. Glad-
stone observed the same, but with him the prismatic image appeared to terminate
at the less refrangible end, just on the C side of B, and at the more refrangible end it
seemed to be suddenly cut off by the line H ; but whether this was precisely the case
could not be determined by measurement, as there was not sufficient light in the field
of view at the extremities of the spectrum to see the cross-wires of the telescope.
When the moon sank towards the horizon, the more refrangible rays were found to be
partially absorbed, while the lines C and D became very strongly marked ; C 6 made its
appearance in its proper position — as determined by angular measurement — and the
band h came distinctly into view.

It is worthy of remark, that there is nothing in the reflecting surface of the moon, or
in her atmosphere (if she have any), which produces fresh bands of absorption in the
solar light, at least as far as prismatic analysis has yet revealed.

Many observers have remarked that there are parts of the prismatic image which give
rise to different sensations of colour according to their intensity. Thus in the spectrum
of direct sunlight a broad space appears yellow, while in that of diffused light the same
colour is sometimes restricted perhaps to the narrow bright band between D and D 2, the
adjoining rays appearing orange on the one side, and green on the other. Striking illus-
trations of this were obtained during the examinations of the spectrum afforded by moon-
light. Sir David Beewstee saw the green space extending a little beyond F; Dr. Glad-
stoke found the portion between G and H, which is usually violet, to be strictly analo-
gous in colour to those " lavender " rays beyond H, which he was then studying ; and
another party, to whom he showed the lunar spectrum, remarked at once the abnormal
colour of these rays, but designated them "grey," and that without knowing that
Sir John Heeschel had applied the same term to the almost invisible extreme rays of
the solar image. In like manner, Sir David Beewstee on one occasion in describing a
spectrum of the western sky after sunset, adds the note, — " There is a slight tinge of
blue close to F, but the general colour beyond it is violet."

# For Eraunhoeer's observations see G-ilbert's Ann. vol. lxxiv. p. 375,

THE LINES OP THE SOLAE SPECTETTM, 157

Lines produced by Absorbent Media.

In the paper already referred to*, Sir David Brewster described the remarkable
series of dark lines and bands which make their appearance in the spectrum when
nitrous acid gas is interposed between the prism and the source of light, whether that
be the sun or a burning lamp. He mentions also the circumstance that heating the gas
produces the same increase in the number and breadth of these lines as an increased
thickness of the gaseous stratum itself does. From his drawings made about that time,
fig. 10 is compiled. It is on half the scale of fig. 1, and the principal lines of the solar
spectrum are inserted, with a view of identifying the position of the nitrous bands.
These bands are numbered in the figure, beginning not with 1 but with 10, so that any
future observer publishing a map with those lines in the orange and red spaces, which
require a greater thickness of gas, or a higher temperature to develope them, may
continue the numbers also in the same backward direction.

Almost immediately after the publication of this effect of nitrous acid fumes on light,
Professor W. H. Miller, of Cambridge, announced the discovery of different series of
lines caused in the spectrum by the interposition of bromine and iodine vapour, and
euchlorine gasf. These lines differ wholly from the preceding. Subsequently Professor
W. A. Miller, of King's College, investigated the subject, and published^ not merely a
description, but coloured drawings also of the lines and bands of absorption produced by
iodine, bromine, chlorous acid, nitrous acid, and perchloride of manganese. His delinea-
tion of the nitrous bands does not profess to be very accurate, and differs considerably
in detail from the much fuller drawings that are united together in fig. 10, partly per-
haps because he employed a sufficiently dense stratum of gas to bring out lines in the
red, and to intercept nearly the whole of the violet rays.

It was observed by Sir David Brewster in the same paper §, that a solution of oxalate
of chromium and potash has the remarkable property of giving rise to a sharp and narrow
black band in the prismatic image. This band coincides with the bright space between
the solar bands a 6 and a 7. Analogous absorption bands have been remarked when light
has been transmitted through solutions of other salts ; for instance, blue compounds of
cobalt, salts of the protoxide and of the peroxide of uranium, permanganate of potash,
and salts of didymium, beside such substances as chlorophyll, alizarine, and purpurine || .

Origin of Lines.
The origin of these fixed lines and bands in the solar spectrum is a question still
unresolved. It may be conceived —

* Phil. Trans. Edinb. vol. xii. p. 522. f Phil - Ma g« 1834 - X Ibid - Au g ust > 1845 > p. 81.

§ P. 525. See also Philosophical Transactions, 1835, p. 92.

|| See the papers of Sir John Herschel, Phil. Trans. Edinb. ix. ; of Sir Dayid Brewster, Phil. Trans.
Edinb. xii. p. 588 ; of Professor Stokes, Philosophical Transactions, 1852, pp. 487, 517, 522, 558, and Quart.
Journ. Chem. Soc. 1859, p. 219 ; and of Dr. Gladstone, Quart. Journ. Chem. Soc. 1 857, pp. 79 and 219,
and Phil. Mag. Dec. 1857.

MDCCCLX. Y

158 SIB DAVID BEEWSTEE AND DE. J. H. GLADSTOlSrE ON

1st. That the light when emitted from the photosphere itself is deficient in these rays.
This was evidently the idea of Fkaujsthofer.

2nd. That they are due to absorption by the sun's atmosphere.

3rd. That they are due to absorption by the earth's atmosphere.
Or it is conceivable that some of these lines and bands have one origin and some
another, though it seems more natural to refer them all to one cause.

The first supposition, that they are originally wanting in the light itself, scarcely
admits of a positive proof.

If the second supposition, that they (or some of them) are due to the sun's atmosphere,
be true, it may be expected that the light which comes through a minimum amount of
such atmosphere would present less appearance of these lines than that which has
traversed a longer portion. During the eclipse of the sun in March 1858, Dr. Glad-
stone made preparations for determining whether the light coming merely from the thin
edge of his disk would be different in this respect from the ordinary sunlight, but unfor-
tunately clouds prevented the experiment being carried out on that occasion*. How-
ever, by other contrivances, each of the authors came independently to the conclusion
that there is no perceptible difference in this respect between the light from the edge
and that from the centre of the solar disk.

That the earth's atmosphere has much to do with the manifestation of these lines is
beyond all question, and the analogy of such gases as nitrous acid, or bromine vapour,
suggests the idea that they may originate wholly in the air that encircles our globe. Nor
does the observation that an increased depth of air affects some lines greatly, and others
little, if at all, militate against the supposition that they are all due to the same
absorbent medium ; for such a band as No. 44 of the nitrous acid spectrum (fig. 10) will
appear much the same whether a thin or a thick stratum of the gas has been interposed,
while the band No. 10 will only make its appearance when the stratum is deep. Again,
a thin stratum of a uranium salt in solution shows a dark band in the greenish-blue
space, and on increasing the thickness this dark band remains almost the same in width,
while four new bands make their appearance in the blue portion, and the violet is
absorbed. This question, however, appears susceptible of an experimentum cruris.
Were we to take any artificial light, and examine it by a prism through a sufficient
length of air, we ought to see the fixed lines developing themselves, if they be due to
the atmosphere, or entirely absent, if otherwise. This was tried by Dr. Gladstone.
The lighthouse at Beachy Head exhibits thirty oil-lamps on a triangular stage, each
furnished with a large parabolic silvered reflector. The light of ten of these lamps at
once is thus thrown into one beam, which as the apparatus revolves sweeps the horizon,
and is visible about as far as any artificial light in existence. On the nights of August

* After this communication had been sent to the Society, Dr. Gladstone was made acquainted with the
fact that the same idea had occurred previously to Professor Forbes, and that he had determined by the
annular eclipse of May 15, 1836, that the light from the edge of the solar disk is identical in this respect
with ordinary sunlight. — Phil. Mag. vol. ix. p. 522.

THE LINES OF THE SOLAE SPECTEUM. 159

31 and September 1, 1859, it was seen from Shoreham and Worthing, at the distance of
twenty-five and twenty-seven miles respectively, and across the sea, like a star of the
second magnitude, but of an orange colour. On examining it by the prism and tele-
scope, it was resolved into a thread of light of a pale yellow colour, shading off into red
at one end and into green at the other, while absorbent glasses showed that the spec-
trum did not extend either way beyond C and F. No lines were detected. The result
therefore is unfavourable to the above supposition; for though twenty-seven miles of
atmosphere, even at its densest, could not be expected to produce what are described
above as "atmospheric lines," D and b might reasonably have been looked for; yet on
account of the faintness of the light, and the difficulty of distinguishing with certainty a
minute break in a flickering luminous thread, no great reliance should be placed on this
negative result. The stars promise the best means of solution, since their light traverses
precisely the same terrestrial atmosphere as that of the sun does. Feaujsthofer saw, as
was antecedently probable, some new lines of absorption in the spectra of certain fixed
stars, while he recognized the line D in Pollux and Procyon, and the lines D and b in
Capella and Betalgeus. Yet he did not observe these familiar lines in the spectra
of Sirius and Castor ; but it may be fairly open to doubt whether this did not arise from
the extreme difficulties attending the observation. The experiments of the present
authors in this direction, though they have had the use of powerful telescopes, have not
led to any definite conclusions as to these lines, but they have both noted other remarkable
phenomena of absorption, especially on examining the spectra of the coloured stars *.

The origin of the fixed lines of the solar spectrum must therefore still be considered
an undecided question f.

Luminous Bands in Artificial Lights.

While the spectra of some artificial lights exhibit all the coloured rays gradually
shading one into the other, those of some other lights consist of a series of luminous
bands separated by dark spaces ; and one of the most remarkable facts is this, that these
luminous bands sometimes coincide with the dark lines of the solar spectrum. Thus the
intense yellow light of the soda-flame is well known to have the same refrangibility as
D ; and this ray is prominent also, according to W. A. Miller, in the flames of lime,
strontia, baryta, zinc, iron, and platinum, and, according to Angstrom, in the electric
flames of every metal examined by him. But the most remarkable case occurs when
carbon or sulphur is burnt in nitre. The brilliant light, when analysed by a prism,
exhibits a spectrum about as long as that of the sun at noon day, but marked by
bright lines, among which three are particularly prominent, respectively violet, yellow,
and red in colour. The violet ray is not quite so refrangible as the solar H, but the
yellow is coincident with D, and the red with A, while between the red and yellow

* Eor Sir D. Brewster's observations see his ' Optics,' Ed. 1853, p. 94.

f "Wherever they originate it is possible that they may be phenomena of interference, as Sir David Brew-
ster has observed analogous lines and bands in portions of decomposed glass consisting of numerous films.

160 ON THE LINES OF THE SOLAR SPECTRUM.

appear at times fainter lines, one of which coincides with B, and a bundle sometimes
appears in about the position of a.

Fig. 11 represents a series of lines, principally orange, that appear in the spectrum of
a spirit-lamp flame, on the wick of which nitrate of strontia has been placed. It is
from a drawing by Sir David Bkewster, and its position in the spectrum is indicated by
the ordinary letters. The yellow line coincident with D is very prominent.

P.S. Much additional light has been thrown on these luminous bands, and their
relation to the dark lines of the solar spectrum, by the recent labours of continental
observers.

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